Extraction of Aromatic Hydrocarbons from Gasoline Fraction by Blended Extractants

Abstract

Extraction of aromatic hydrocarbons from the 62-180°C gasoline fraction of Astrakhan gascondensate is studied for the preparation of pyrolysis feedstock. Comparative analysis of the extraction capacity of triethylene glycol, N-methylpyrrolidone, and blended extractants showed that a blended extractant containing 40-50 wt. % polyethylene glycol, 45-55% N-methylpyrrolidone, and 5% water is most effective in extracting aromatic hydrocarbons.Pyrolysis of the gasoline fraction is one of the basic processes employed to produce lower olefins, the yield of which is affected by a number of factors, including the hydrocarbon composition of the pyrolysis feedstock. It is established that the aromatic hydrocarbons contained in the feedstock do not participate in decomposition reactions, resulting in the formation of gaseous hydrocarbons [1]. Moreover, a high content of aromatic hydrocarbons in the pyrolysis feedstock will lead to carbon deposition in the reactors and to a reduction is the output of the production plants. The need therefore arises for preliminary extraction of aromatic hydrocarbons from the pyrolysis feedstock.Ethylene glycol, diethylene glycol (DEG), triethylene glycol (TEG), N-methylpyrrolidone (NMP),acetonitrile, pentane, acetone, etc. are widely used in industry to extract aromatic hydrocarbons in blends with non-aromatic hydrocarbons [2]. In Russia, DEG and TEG are in greatest demand as extractants in industrial processes rendering aromatic hydrocarbons. The extractants are characterized by low selectivity and dissolving power, indicating that the extraction is conducted at high ratios of extractant to feedstock, and temperature. This, in turn, gives rise to an increase in the cost of the final products. A. A. Gaile and collaborators [3-6] suggest that blended extractants be used to solve this problem. Conversion of production plants to blended extractants does not require their reconstruction. 231 Table 1 Composition of extractant, wt. % Toluol content in refined oil, wt. % Degree of toluol extraction, % TEG NMP water -95 5 13.8 54.0 10 85 5 12.9 57.0 20 75 5 12.5 58.3 30 65 5 10.4 65.3 40 55 5 9.3 69.0 50 45 5 9.0 70.0 60 35 5 10.8 64.0 70 25 5 11.0 63.4 80 15 5 11.4 62.0 90 5 5 12.0 60.0 95 -5 14.8 50.7The purpose of this study is to select an extractant that ensures maximum extraction of aromatic hydrocarbons from the 62-180°C gasoline fraction to prepare pyrolysis feedstock, and produce an extract with a high content of aromatic hydrocarbons.The 62-180°C fraction of Astrakhan gas condensate, which contains up to 37 wt. % aromatic hydrocarbons served as feedstock. Blends of TEG and NMP were used as extractants, since the blended extractant is distinguished by elevated dissolving power and selectivity with respect to aromatic hydrocarbons as compared with the pure TEG [7].Single-stage extraction with use of a mock-up blend containing 30 wt. % toluol and 70 wt. % n-heptane as feedstock was conducted to compare the extractants with respect to extraction capacity. Extractions were carried out in thermost...